1. Field of the Invention
The invention relates to a failure diagnostic apparatus that is provided for a canister system for processing evaporated fuel generated in a fuel tank (e.g. a fuel tank for an automobile), and a failure diagnostic method that is performed by the failure diagnostic apparatus.
2. Description of the Related Art
As disclosed in Japanese Patent Application Publication No. JP(A) 2003-28009, a canister system for preventing evaporated fuel generated in a fuel tank from being released in the atmosphere is provided in a fuel supply system of an engine for an automobile. In such a system, the evaporated fuel generated in the fuel tank is temporarily stored in a canister, and the evaporated fuel is introduced (purged) to an intake passage by an intake negative pressure of an intake system of the engine.
In this type of system, if a failure, for example, formation of a hole, a crack, and inappropriate sealing, occurs in the fuel tank or the canister, the evaporated fuel leaks from a portion where the failure occurs. It is, therefore, important to detect such a failure promptly in this type of system.
Generally, a following failure diagnostic operation is performed in order to detect such a failure. In this operation, first, a negative pressure (an intake negative pressure of an engine intake system) is introduced to a system including the fuel tank and the canister, while the system is shut off from the outside air. Then, when the pressure in the system has reached a predetermined negative pressure, introduction of the negative pressure is stopped (i.e, the system is sealed). A change in the pressure in the system, which occurs after the introduction of the negative pressure is stopped, is monitored by a pressure sensor. If a failure has occurred, the pressure in the system increases to a value close to atmospheric pressure. On the other hand, when a failure has not occurred, the pressure in the system is maintained at a negative pressure or increases by a considerably small amount. Accordingly, monitoring a change in the pressure in the system makes it possible to determine whether a failure has occurred.
Meanwhile, as disclosed in, for example, Japanese Patent Application Publication No. JP(A) 09-195861, an in-tank type of canister system in which a canister is housed in a fuel tank (hereinafter, simply referred to as an “in-tank canister system”) has been used recently. In the in-tank canister system, a major portion of a pipe can be located in the fuel tank. Accordingly, the in-tank canister system has an advantage that, even if evaporated fuel leaks from the pipe, a joint thereof, or the like, the evaporated fuel is not released into the atmosphere.
However, in this in-tank canister system, even if a failure (formation of a hole, or the like) occurs in the canister, the failure cannot be detected by performing the above-mentioned failure diagnostic operation. When the above-mentioned failure diagnostic operation is performed on the in-tank canister system, the canister is placed under a negative pressure in the fuel tank. Accordingly, even if a failure has occurred in the canister, when a failure has not occurred in the fuel tank, the pressure in the system does not increase to atmospheric pressure.
In order to address such a problem, Japanese Patent Application Publication No. JP(A) 2001-115915 discloses a technology for discriminating between a failure in a fuel tank and a failure in a canister.
In a failure diagnostic method disclosed in Japanese Patent Application Publication No. JP(A) 2001-115915, a purge passage through which evaporated fuel in the canister is introduced into an intake passage, and an evaporation passage (an evaporated fuel introduction passage) through which evaporated fuel in the fuel tank is introduced into the canister are connected to each other by a branch pipe, and a three-way valve is provided at a portion at which the branch pipe is connected to the purge passage. First, communication between the inside of the fuel tank and the purge passage through the branch pipe is permitted by changing the state of the three-way valve, and communication between the inside of the canister and the atmosphere is permitted by a new air introduction passage. Thus, a negative pressure is introduced into the fuel tank, while the pressure in the canister is maintained at atmospheric pressure. If the pressure in the fuel tank does not reach a target negative pressure even when a predetermined period has elapsed, or if the purge passage is blocked after the target negative pressure is reached and the pressure in the fuel tank gradually increases (the pressure in the fuel tank increases since the negative pressure in the fuel tank leaks to the inside of the canister when a failure occurs in the canister), it is determined that a failure has occurred (e.g. a hole is formed in the fuel tank or the canister, that is, there is a leakage) (i.e., a leakage diagnostic operation is performed).
Further, communication between both the inside of the fuel tank and the inside of the canister, and the purge passage is permitted through the branch pipe and the evaporation passage, and a negative pressure is introduced into the fuel tank and the canister. If the pressure in the fuel tank does not reach the target negative pressure even when a predetermined period has elapsed, or if the purge passage is blocked after the target negative pressure is reached and the pressure in the fuel tank increases to a value close to atmospheric pressure, it is determined that a failure has occurred in the fuel tank. On the other hand, if an amount of change in the pressure in the fuel tank is small, it is determined that a failure has occurred in the canister (i.e., a leakage portion diagnostic operation is performed).
However, according to the failure diagnostic method disclosed in Japanese Patent Application Publication No. JP(A) 2001-115915, it is necessary to provide the branch pipe for connecting the purge passage to the evaporation passage, and to provide the three-way valve at the portion at which the branch pipe is connected to the purge passage. Many extra components such as the branch pipe and the three-way valve need to be provided, resulting in a complicated structure and an increase in production cost.
In the “leakage portion diagnostic operation”, when a failure has occurred in the fuel tank, the pressure in the fuel tank does not reach the target negative pressure even when the predetermined period has elapsed, or the pressure in the fuel tank increases to a value close to atmospheric pressure when the purge passage is blocked after the target negative pressure is reached, regardless of whether a failure has occurred in the canister. Namely, when a failure has occurred in the fuel tank, whether a failure has occurred in the canister cannot be determined. In other words, it is impossible to discriminate between the state where “a failure has occurred in both the fuel tank and the canister” and the state where “a failure has occurred in the fuel tank, but a failure has not occurred in the canister”.
As described so far, concerning the in-tank canister system, a technology for accurately discriminating between a failure in the fuel tank and a failure in the canister has not been established.